The current resulting from the opening and closing of single acetylcholine (ACH) activated ionic channels was studied in sympathetic neurons of bullfrog by the patch clamp technique with gigaohm seals. Single channel current was linearly related to the potential across the patch and had an extrapolated reversal potential near 0 mV. The single channel cord conductance was 30 ps. At all membrane potentials studied the channel open time was a single exponential function with a time constant equal to the mean open time. Occasionally, deviations from exponential distributions were observed. Changes in patch potential produced no consistent change in channel open time.